An undisturbed peat bog at Bad a' Cheo in the Caithness region of the Scottish Highlands. Photograph: Peter Hulme/Corbis

Let's be clear: Britain needs wind turbines. Lots of them. But just about the worst place to erect them is on top of peat bogs, which are huge stores of carbon that can easily leak carbon dioxide into the air when damaged by the inevitable roads or drains.

More than half of the wind turbines in Scotland are on highland peat. This is not sensible. Scottish peat bogs hold three-quarters of all the carbon in British ecosystems – equivalent to around a century of emissions from fossil fuel burning.

Apart from water, peat bogs are largely composed of huge volumes of saturated, undecayed plants. A single hectare typically contains more than 5000 tonnes of carbon, ten times more than a typical hectare of forest. But any disturbance leads to lower water levels and to the peat drying, oxidising and releasing its carbon, says biochemist Mike Hall of the Cumbria Wildlife Trust.

The bog can decompose for hundreds of metres round every turbine, potentially releasing millions of tonnes of carbon. The process is slow, but frequently unstoppable, Hall says. So many wind farms may eventually emit more carbon than an equivalent coal-fired power station.

Is that the case on the Shetlands project, which will have 150 giant turbines and 118 kilometres of roads, most of them on deep bog? The promoters, Viking Energy, say the "payback time" for the turbines – that is, the time they will have to run before they recoup the carbon emissions from peat loss – could be as little as 2.3 years, or as much as 14.9 years. The higher figure is three-fifths of the assumed 25-year lifetime of the wind farm.

But dig deeper and even this high figure seems little better than guesswork.

An appendix in the project's environmental statement shows that just 10 out of 69 criteria are responsible for the difference between the best and worst-case scenarios of carbon loss. The criteria cover things such as how much peat would be drained, and how much the water table would fall as a result. But, worryingly, none of those 10 criteria were backed up by site data. The input figures for each were "assumed values".

Moreover, some critical input data that did not vary between the best and worst cases also seemed somewhat arbitrary. Thus the time required for the bog to stop leaking carbon after the closure of the site and the blocking of drains was set at 10 years. Why ten years? This is described as a "default value". Not reassuring.

But that hasn't stopped Viking's environmental statement from stating that the risk of a slide, even in a worst-case scenario, is zero. It blandly states: "It has been assumed that measures have been taken to may [sic] limit damage so that C losses due to peat landslide can be assumed to be negligible."

I asked David Thomson, the project officer for Viking Energy, about the veracity of these payback calculations. He said: "It's not perfect, but as a developer we submit a defendable ranged estimate using an accepted methodology and then it is for others to judge ... Ultimately it is a model. It has calculations. The quality of the answer is entirely subject to the initial inputs."

I appreciate that candour. But, much as we need more wind turbines to harness one of our most valuable natural resources, I think we deserve better information than that before deciding where to put them. When erecting wind turbines on the nation's largest carbon store, we need estimates of the likely carbon loss that are more than simply "defendable", and are not "entirely subject" to "assumed values".